LEDs are typically mounted on a submount wafer that is later diced to separate out the individual LEDs/submounts. Each submount portion of the wafer has top electrodes that are bonded to electrodes on the LED, such as by ultrasonic bonding. An underfill material, such as epoxy or silicone, is then injected under the LED to provide mechanical support and protect the LED from contaminants.
The submount also has a set of more robust electrodes, electrically connected by a metal pattern to the LED electrodes, that are typically bonded to a printed circuit board (after the submount wafer is diced) using conventional solder reflow or other means.
It is known to provide large reflective metal electrodes, such as silver, on the bottom surface of an LED so that light emitted downward by the LED active layer is reflected upward rather than being absorbed by the submount. Forming such reflective metal electrodes can be complex. The preparation of the semiconductor surface, deposition and pattering of the silver mirror, and encapsulation with guard sheets to prevent metal intermixing or contamination can be a very complicated and delicate process that has to be closely monitored.
The LED can generate a lot of heat, and the heat may be conducted through the submount and into a heat sink. Most of the heat may be conducted through the LED metal electrodes to the submount. Silicone and epoxy underfill materials are poor conductors of heat.
What is needed is a better way to reflect light upwards from the surface of a submount and increase the removal of heat from the LED.